Plastic foams are used for various applications because, in accordance with the materials and the foamed state of the plastic foams, various functions such as heat insulation, heat shielding, sound insulation, sound absorption, vibration proof, vibration damping, and weight saving can be expressed. An exemplary method for producing plastic foams include the steps of: molding a resin composition containing a foaming agent or foamable masterbatch and a matrix resin (e.g. thermoplastic resin) by a molding method such as injection molding and extrusion molding; and foaming the foaming agent by heat generated in molding.
For production of the plastic foams, thermally expandable microcapsules are used as a foaming agent, which includes thermoplastic shell polymers containing a volatile expansion agent that is gasified at a temperature lower than the softening point of the shell polymers. Such thermally expandable microcapsules are expanded by gasification of the volatile expansion agent and softening of the shell polymers by heating.
Patent Literature 1, for example, discloses a thermally expandable microcapsule having: an outer shell including polymers obtained by polymerization of a monomer mixture that contains a nitrile monomer (I), a monomer (II) having one unsaturated double bond and a carboxyl group in a molecule, a monomer (III) having two or more polymerizable double bonds in a molecule, and, if needed, a monomer (IV) different from and copolymerizable with the monomers (I), (II), and (III); and a foaming agent enclosed in the outer shell.
Patent Literature 1 teaches that the thermally expandable microcapsule disclosed therein stably foams in a high temperature range at a high expansion ratio and the resulting foam can serve as a highly elastic product.
However, conventional thermally expandable microcapsules as disclosed in Patent Literature 1 may have been soon deflated by outgassing of the volatile expansion agent and burst or shrinkage of shell polymers, which indicates such thermally expandable microcapsules are still insufficient in terms of the heat resistance and durability.
Patent Literature 2 discloses a thermally foamable microsphere. In the microsphere, an outer shell containing a foaming agent can be a copolymer having a polymethacrylimide structure, and monomers forming the polymethacrylimide structure by the copolymerization reaction are methacrylonitrile and methacrylic acid.
Patent Literature 2 teaches that the thermally foamable microsphere disclosed therein is excellent in heat resistance and stably foams at a high expansion ratio because the outer shell therein is a copolymer which can form a polymethacrylimide structure.